In many industrial environments, it is necessary to detect the level of product in a holding tank or bin. Level sensors are typically attached to the holding tank or bin, and electrically connected to remote gauges at a control room or other central location, where technicians or control systems may monitor the status of the bins to provide the appropriate process control. Various technologies have been developed for level sensing. These include various contact sensing technologies using floats or drop weights, as well as various non-contact technologies, such as reflecting electromagnetic radiation or ultrasonic vibrations from the surface of the product in the bin to determine the height of the product. In some applications, it is particularly important to move the sensor away from the product. For example, in a foundry where the level of a hot melt of steel or ore is to be level sensed, it is particularly important to keep the level sensor a safe distance from the hot melt. In these applications, nuclear level sensing gauges are used.
In a nuclear level sensing gauge, a source of nuclear radiation is positioned on one side of the bin to be level sensed. A nuclear radiation detector is placed on the opposite side of the bin. The radiation exiting the source is in the shape of a wide, generally vertically dispersed beam, directed toward the interior of the bin. The product in the bin substantially absorbs the radiation that impinges upon it. If, however, the bin is not full of product, some part of the beam of radiation from the source passes through the bin and irradiates the radiation detector on the opposite side of the bin from the radiation source. Because the product in the bin substantially absorbs the radiation that impinges upon it, thus reducing the amount of the radiation beam passing through the bin, the amount of radiation stimulating the radiation detector is inversely proportional to the amount of product in the bin. The radiation reaching the detector creates scintillating light flashes in the detector. The number of light flashes is proportional to the intensity of the incident radiation. A high sensitivity light sensor converts the light flashes into electrical pulses, which are amplified and evaluated by electronics to produce a measurement of the amount of product in the bin.
Traditionally, nuclear level sensing gauges have used an elongated scintillating crystal as a radiation detector. The scintillating crystal produces photons of light when exposed to nuclear radiation from a radiation source. The number of photons produced is related to the amount of radiation impinging on the crystal. A photomultiplier tube (PMT), used as a light sensor, is coupled to an end of the crystal. The PMT detects photons of light emanating from the scintillating crystal, and produces a signal indicative of the amount of radiation impinging on the crystal and, thus, the level of product in the bin. This type of sensor is discussed in U.S. Pat. Nos. 3,884,288, 4,481,595, 4,651,800, 4,735,253, 4,739,819 and 5,564,487.
Nuclear level sensing gauges have also been developed which utilize a bundle of one or more scintillating fibers as the radiation detector in place of a scintillating crystal. The scintillating fiber bundle may be directly coupled to a PMT, or coupled to a PMT via a light guide, which permits the PMT and amplifying electronics to be positioned remotely from the fiber bundle. The use of scintillating fibers yields substantial improvements in cost, performance, and ease of use; as well as size and sensitivity, as compared to gauges which use a scintillating crystal. Specifically, compared to a scintillating crystal, the scintillating fibers are lightweight, can be easily coiled for shipment, and are easily cut to the desired lengths. Scintillating fibers can be readily curved to match the curvature of a particular bin, whereas crystals are rigid and difficult to custom manufacture. Also, scintillating fibers have better internal reflection characteristics than crystals, meaning that fiber scintillating detectors can be made longer with less loss than crystal scintillating detectors. Finally, a bundle of one or more fibers can have substantially less heat capacity than the corresponding crystal, meaning the bundle is more readily cooled.
Unfortunately, both crystals and fibers exhibit light intensity losses when manufactured in long lengths. FIG. 1 illustrates the decay of light intensity as a function of the distance of travel from a scintillation source through a medium, and the definition of the “attenuation length” L (1/e) of a medium, which is defined as the distance that light can be transmitted through a medium before the light intensity is reduced to lie of its intensity at its origin. A fiber bundle typically has an attenuation length of about 2.5 meters. As can be seen from the FIG. 1 curve of light intensity vs. distance of travel, light loss is relatively severe at distances longer than the attenuation length, and nonlinear. However, fiber bundles and crystals have been used commercially at long lengths, up to 10 feet for crystals and 12 feet or longer for fiber bundles. Crystals are practically limited to approximately 10 foot lengths because of the difficulty of manufacturing bars in longer sizes. Fibers are not practically limited by manufacturing constraints, but are constrained by the attenuation length of the polystyrene medium used to make the fibers.
Engineers, confronting the limited lengths of scintillating crystals, have created serialized devices that use multiple crystals for level sensing. FIG. 2 shows a typical prior art arrangement of this kind, in which a plurality of scintillating crystals 14 are placed in a serial fashion adjacent a bin opposite to a radiation source S, each crystal stimulating a photomultiplier tube 12 which is coupled to electronic amplifiers 10. The output of the various amplifiers 10 is then coupled to summation electronics 20. Each crystal 14 has a length less than the attenuation length of the crystal medium, but the serially positioned crystals have a collective length Lt that can be substantially greater than the attenuation length. FIG. 3 shows an alternative, serialized arrangement of crystals 14 that has been used in installations where it is desired to move the photomultiplier tubes 12 remote from the crystals 14. In this arrangement, a light guide 18 couples light from each crystal 14 to each PMT 12. As in FIG. 2, the crystals are generally cut to a length less than the attenuation length of the crystal medium, but have a collective length Lt that can be substantially longer. While the arrangements illustrated in FIGS. 2 and 3 facilitate longer length applications, these gauges are highly complex and costly, due to the replication of the PMT 12 and electronics 10, and the requirement for a summation electronics unit 20.
In order to overcome the problems, cost, and complexity of prior nuclear level sensing gauges, Applicants, in U.S. patent application Ser. No. 13/798,179, disclose the use of a nuclear level sensing gauge having a plurality of scintillators, positioned in a serial fashion, adjacent the product in a bin. The scintillators are positioned on the opposite side of the bin from the nuclear source. As described therein, and shown in FIG. 4, the level sensing gauge uses a plurality of light guides 18 to couple light from the scintillators (either crystals 14 or fiber bundles 16) to a common light sensor or PMT 12, so that the common light sensor detects light generated in two or more scintillators. Accordingly, the number of photons generated in the scintillators is measured by a single light sensor, producing a measure of radiation-absorbing product in the bin, without the cost and complexity of multiple PMTs and amplifying electronics, as in the previous gauges.
While the use of a single, common light sensor greatly reduces the cost and complexity of the level sensing gauge, the rigidity of crystals 14 and light guides 18 in the gauge shown in FIG. 4 can limit the available placement of the gauge relative to bins or vessels having curved or complex shapes. Heretofore, gauges have typically been enclosed in a rigid housing, such as stainless steel, to provide protection from harsh operating conditions, and prevent movement or distortion in the optical couplings between the scintillator, light guide and light sensor. Additionally, in the prior gauges intimate contact has been required between the scintillator and light guide. This coupling contact has been required to facilitate collection of as many of the scintillation light flashes as possible, while reducing losses at the optical interfaces. Light guides have been cemented or otherwise securely affixed to the scintillators to optically couple the scintillators and light guides. However, tight, durable contact couplings can be difficult to assemble in the field, due to the harsh operating environment of many nuclear gauge applications. Additionally, the contact couplings in the light path may degrade over time due to movement or the harsh environment, adversely affecting operation of the gauge.
Measurement applications also may dictate a more flexible gauge in order to conform the gauge to the curvature of the bin or vessel being measured. For example, it can be desirable for the gauge to spiral down the exterior of a vessel in order to more accurately measure the contents of the vessel. While gauges have been developed with some degree of flexibility, this flexibility has been limited due to the need to maintain coupling contact between the scintillator and light guide. Prior gauges have attempted to couple the scintillator and light guide through an air gap, but the air gap can fill with water and freeze, causing refraction of the light and loss of output. Accordingly, to accommodate vessels of various shapes, while maintaining a high sensitivity profile, it is desirable to have a nuclear level sensing gauge with increased flexibility that can bend in three dimensions in order to conform the gauge to the shape of the product bin or vessel. Additionally, it is desirable to have a nuclear level sensing gauge that does not require coupling contact between the scintillator and light guide in order to transfer light from the scintillating detectors to the light sensor.